CVE-2026-92240: Thunderbird IMAP Parser DOS Vulnerability

CVE-2026-92240 Overview

CVE-2026-92240 is an out-of-bounds read vulnerability in the Internet Message Access Protocol (IMAP) response parser used by Mozilla Thunderbird. A malicious or compromised IMAP server can trigger the flaw by sending an untagged * ID response, causing the client to crash. The affected parsing path is reachable before authentication, meaning attackers do not need valid credentials to reach the vulnerable code. Mozilla addressed the issue in Thunderbird 156, Thunderbird 140.16, and Thunderbird 153.3.

Critical Impact
A hostile IMAP server can crash Thunderbird clients without authentication by returning a malformed untagged * ID response, enabling denial-of-service against mail users and potentially exposing adjacent process memory during the out-of-bounds read.

Affected Products

Mozilla Thunderbird versions prior to 140.16 (ESR branch)
Mozilla Thunderbird versions prior to 153.3
Mozilla Thunderbird versions prior to 156

Discovery Timeline

2026-09-15 - CVE-2026-92240 published to the National Vulnerability Database (NVD)
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-92240

Vulnerability Analysis

The vulnerability resides in the routine that parses IMAP server responses inside Thunderbird's mail networking stack. When the server issues an untagged capability-style response prefixed with * ID, the parser reads beyond the bounds of the response buffer. This out-of-bounds read causes the client process to terminate, disrupting mail retrieval and background synchronization.

The parsing path executes during the initial protocol handshake, before Thunderbird submits credentials. Any TCP endpoint that Thunderbird connects to as an IMAP server can therefore trigger the crash, regardless of whether the user has an account on that server.

Details on the underlying defect are tracked in Mozilla Bug Report #2069113 and the corresponding Mozilla Foundation Security Advisories MFSA-2026-94, MFSA-2026-95, and MFSA-2026-96.

Root Cause

The defect is an out-of-bounds read [CWE-125] in the IMAP response parser. The parser fails to validate the length or structure of the untagged * ID response before dereferencing bytes from the response buffer. When the server-controlled data does not conform to the parser's expected layout, the read reaches memory outside the allocated buffer.

Attack Vector

The attacker must control an IMAP server that a Thunderbird client connects to. This can be achieved by hosting a hostile server, compromising a legitimate mail server, or performing a network-level machine-in-the-middle attack against unencrypted or improperly validated IMAP sessions. Once the client initiates a session, the server returns a crafted untagged * ID line to trigger the out-of-bounds read and crash the client.

Because the flaw is reached before authentication, a user only needs to configure or connect to a mailbox on the malicious endpoint. No user interaction beyond the normal mail check cycle is required. See the referenced Mozilla advisories for further technical context.

Detection Methods for CVE-2026-92240

Indicators of Compromise

Repeated, unexpected crashes of the Thunderbird process (thunderbird.exe on Windows, thunderbird on Linux, thunderbird-bin on macOS) shortly after connecting to an IMAP account.
IMAP sessions in packet captures containing untagged server responses beginning with * ID and unusual or malformed payload structures.
Crash reports in the operating system's application error log referencing the Thunderbird IMAP networking module.

Detection Strategies

Inspect network telemetry for IMAP traffic to servers outside sanctioned mail providers, particularly on ports 143 and 993.
Correlate Thunderbird process termination events with preceding outbound IMAP connections to identify server-triggered crashes.
Monitor endpoint software inventory for Thunderbird builds older than 140.16, 153.3, or 156 and flag them for patching.

Monitoring Recommendations

Enable operating system crash reporting and forward crash artifacts to a central location for review.
Track outbound IMAP session establishment against an allowlist of approved mail hosts.
Alert on user configuration changes that add new IMAP accounts pointing to previously unseen servers.

How to Mitigate CVE-2026-92240

Immediate Actions Required

Upgrade Thunderbird to version 140.16, 153.3, or 156, matching the release channel deployed in your environment.
Restrict Thunderbird IMAP account configuration to trusted mail providers using group policy or configuration management.
Enforce TLS with strict certificate validation on IMAP connections (port 993) to reduce exposure to machine-in-the-middle servers.

Patch Information

Mozilla fixed the vulnerability in Thunderbird 156, Thunderbird 140.16 (Extended Support Release), and Thunderbird 153.3. Administrators should deploy the release that matches their supported channel. Refer to MFSA-2026-94, MFSA-2026-95, and MFSA-2026-96 for the full advisory text and release notes.

Workarounds

Disable or remove IMAP account configurations that point to untrusted or unverified servers until patches are applied.
Block outbound connections to unapproved IMAP endpoints (ports 143 and 993) at the perimeter or host firewall.
Instruct users to avoid adding new IMAP accounts from unverified sources during the remediation window.

bash

# Example: block outbound IMAP to non-approved hosts on Linux with nftables
nft add rule inet filter output tcp dport {143, 993} \
  ip daddr != { 198.51.100.10, 198.51.100.11 } drop